1. Field of the Invention
The present invention relates to an image display device using, for example, an organic electroluminescence (EL) element, and more particularly, to an image display device capable of displaying a high-quality image with high definition at low voltage.
2. Description of the Related Art
In recent years, the demand has been increased for flat panel display devices instead of cathode ray tube (CRT) display devices, which are the mainstream of conventional display devices. In particular, an organic EL display device using an organic EL element such as an organic light-emitting diode (OLED) is excellent in terms of power consumption, weight, thickness, video image characteristic, view angle, and the like, and hence the development and practical use are advanced.
In the organic EL display device, each pixel has a driving transistor for driving the organic EL element. When a fluctuation in threshold voltage Vth of the driving transistor of each pixel is large, a fluctuation in light emission characteristic of each pixel occurs to reduce the uniformity of a screen, and hence high quality cannot be maintained.
The driving transistor for driving the organic EL element is normally a thin film transistor. The thin film transistor has a large fluctuation in threshold voltage Vth.
Therefore, the organic EL display device has the following problem. That is, the fluctuation in threshold voltage Vth of the driving transistor of each pixel becomes larger, and hence the fluctuation in light emission characteristic of each pixel occurs to reduce the uniformity of the screen. Thus, high quality cannot be maintained.
In view of this point, it is necessary for the organic EL display device to cancel the fluctuation in threshold voltage Vth of the driving transistor of each pixel.
An image display device in which the fluctuation in threshold voltage Vth of the driving transistor of each pixel is cancelled is disclosed in, for example, R. M. A. Dawson et al., “Design of an Improved Pixel for a Polysilicon Active-Matrix Organic LED Display,” SID Symposium Digest 29 11 (1998).
FIG. 13 is a circuit diagram illustrating an equivalent circuit of an example of a pixel of a conventional organic EL display device, which illustrates an equivalent circuit of a most typical pixel employing a voltage program system.
A signal line 12, a selection switch line Y, and a power supply line 6 are led to a pixel 1 illustrated in FIG. 13.
The pixel 1 includes an organic electroluminescence element (hereinafter referred to as organic EL element) 2 serving as a light emitting element.
The organic EL element 2 has a cathode electrode connected to a common ground line, and an anode electrode connected to the power supply line 6 through a p-type thin film transistor (hereinafter referred to as driving TFT) 4.
A holding capacitor element 3 is connected between a gate electrode and a source electrode of the driving TFT 4. The gate electrode of the driving TFT 4 is connected to the signal line 12 through a selection switch element 32 consisting of an n-type thin film transistor. A gate electrode of the selection switch element 32 is connected to the selection switch line Y.
FIG. 14 is a time chart for illustrating an operation of the pixel 1 illustrated in FIG. 13.
In the organic EL display device having the pixel 1 illustrated in FIG. 13, one frame period (FRM) includes a write period and a light emission period. An image voltage is written into the pixel 1 during the write period. Light is emitted for display during the light emission period. The writing of the image voltage is performed for each display line, that is, for each selection switch line Y.
First, as illustrated in FIG. 14, during the write period, the selection switch element 32 is turned on. Then, an analog image voltage is supplied from the signal line 12, and the image voltage is input to the holding capacitor element 3.
During the light emission period, the organic EL element 2 emits light. During the light emission period, the selection switch element 32 is turned off, and hence a voltage corresponding to the image voltage which has been stored in the holding capacitor element 3 is applied to the gate electrode of the driving TFT 4, with the result that a current corresponding to the applied voltage flows through the organic EL element 2, to thereby adjust light emission luminance.
FIG. 15 is a circuit diagram illustrating an equivalent circuit of another example of a pixel of a conventional organic EL display device, and the pixel illustrated in FIG. 15 is a most typical pixel employing a voltage program system.
A signal line 12, a reset line 7, a selection switch line Y, a lighting switch line 21, and a power supply line 6 are led to the pixel 1 illustrated in FIG. 15.
The pixel 1 includes an organic EL element 2 serving as a light emitting element.
The organic EL element 2 has a cathode electrode connected to a common ground line, and an anode electrode connected to the power supply line 6 through a lighting switch element 20 consisting of an n-type thin film transistor and a driving TFT 4 consisting of an p-type thin film transistor.
A second holding capacitor element 30 is connected between a gate electrode and a source electrode of the driving TFT 4. A reset switch element 5 consisting of an n-type thin film transistor is provided between a drain electrode and the gate electrode of the driving TFT 4. Further, the gate electrode of the driving TFT 4 is connected to the signal line 12 through a first holding capacitor element 3 and a selection switch element 32 consisting of an n-type thin film transistor.
A gate electrode of the reset switch element 5 is connected to the reset line 7. A gate electrode of the selection switch element 32 is connected to the selection switch line Y. A gate electrode of the lighting switch element 20 is connected to the lighting switch line 21.
In the organic EL display device having the pixel 1 illustrated in FIG. 15, one frame period (FRM) includes a write period and a light emission period. An image voltage is written into the pixel 1 during the write period. Light is emitted for display during the light emission period. The writing of the image voltage is performed for each display line, that is, for each selection switch line Y.
FIG. 16 is a time chart for illustrating an operation of the pixel 1 illustrated in FIG. 15.
In the following, an operation during each of the write period and the light emission period is described.
First, as illustrated in FIG. 16, during a period between a time t1 and a time t2 of the write period, the reset switch element 5 and the lighting switch element 20 are turned on. As a result, the driving TFT 4 has a diode connection in which the gate electrode is connected to the drain electrode, and hence a voltage of the gate electrode of the driving TFT 4 which has been stored in each of the holding capacitor elements (3 and 30) in a preceding field is cleared.
Next, when the lighting switch element 20 is turned off at the time t2, the driving TFT 4 and the organic EL element 2 forcedly become a current off state. At this time, the gate electrode and the drain electrode of the driving TFT 4 are short-circuited through the reset switch element 5, and hence the voltage of the gate electrode of the driving TFT 4, the gate electrode also corresponding to one end of the first holding capacitor element 3, is automatically reset to a voltage (VDD-Vth) which is lower than a voltage VDD of the power supply line 6 by a threshold voltage Vth.
During a period between the time t1 and the time t3, a fixed voltage (reference voltage) is supplied to the signal line 12. Further, the selection switch element 32 is turned on during the period between the time t1 and the time t3. Therefore, the fixed voltage (reference voltage) is input from the signal line 12 to the other end of the first holding capacitor element 3.
Next, at the time t3, the reset switch element 5 is turned off. After that, an analog image voltage is supplied to the signal line 12, and the image voltage is input to the other end of the first holding capacitor element 3.
During the light emission period starting from the time t5, the reset switch element 5 and the selection switch element 32 are turned off and the lighting switch element 20 is turned on, and hence the organic EL element 2 emits light.
During the light emission period, a voltage corresponding to the change from the reference voltage to the image voltage is applied to the gate electrode of the driving TFT 4, and hence a current corresponding to the applied voltage flows through the organic EL element 2, to thereby adjust light emission luminance.
As described above, in each pixel 1 of the organic EL display device illustrated in FIG. 15, the voltage of the gate electrode of the driving TFT 4 is automatically reset to the voltage (VDD-Vth) which is lower than the voltage VDD on the power supply line 6 by the threshold voltage Vth. Therefore, the fluctuation in threshold voltage of the driving TFT 4 is suppressed, and hence the light emission with high uniformity may be realized.